1. Field of the Invention
This invention relates to a semiconductor device and a semiconductor light emitting device, and in particular, those using nitride III-V compound semiconductors.
2. Description of the Related Art
Nitride III-V compound semiconductors represented by gallium nitride (GaN) (hereinafter also called xe2x80x9cGaN semiconductorsxe2x80x9d) are hopeful materials of light emitting devices capable of emitting light in the green to blue and ultraviolet ranges, high-frequency electronic devices and environment-enduring electronic devices. Especially since light emitting diodes (LED) using GaN semiconductors were brought into practice, GaN semiconductors have become a center of attraction. Realization of semiconductor lasers using GaN semiconductors was also reported, and their application to various purposes, starting from the light source of an optical disc device, is expected.
There is known a GaN semiconductor laser having an AlGaN/GaN/GaInN SCH (separate confinement heterostructure) structure which includes a cladding layer of AlGaN, optical guide layer of GaN and active layer of GaInN. FIG. 1 shows a GaN semiconductor laser of this conventional type.
As shown in FIG. 1, in the conventional GaN semiconductor laser, sequentially stacked on a c-plane sapphire substrate 101 are, via an undoped GaN buffer layer 102 by low-temperature growth: an undoped GaN layer 103, n-type GaN contact layer 104, n-type Al0.07Ga0.93N cladding layer 105, n-type GaN optical guide layer 106, active layer 107 having quantum well layers of undoped Ga0.9In0.1N, p-type Al0.2Ga0.8N cap layer 108, p-type GaN optical guide layer 109, p-type Al0.07Ga0.93N cladding layer 110 and p-type GaN contact layer 111.
Upper part of the n-type GaN contact layer 104, n-type Al0.07Ga0.93N cladding layer 105, n-type GaN optical guide layer 106, active layer 107, p-type Al0.2Ga0.8N cap layer 108, p-type GaN optical guide layer 109, p-type Al0.07Ga0.93N cladding layer 110 and p-type GaN contact layer 111 have the shape of a stripe extending in a direction with a predetermined width.
On the p-type GaN contact layer 111, a stripe-shaped p-side electrode 112 such as Ni/Pt/Au electrode or Ni/Au electrode is provided, and on the n-type GaN contact layer 104 in the region adjacent to the stripe portion, an n-side electrode 113 such as Ti/Al/Pt/Au electrode is provided.
According to the knowledge of the Inventor, it has been confirmed that, from the viewpoint of realizing continuous oscillation of a GaN semiconductor, it is sufficient that the band gap between its cladding layer and active layer is not less than 500 meV. However, in conventional AlGaN/GaN/GaInN SCH-structured GaN semiconductor lasers, if the Al composition of the cladding layer is increased for the purpose of increasing the band gap between the cladding layer and the active layer, their growth becomes difficult. Additionally, since the p-type carrier concentration decreases in AlGaN with a high Al composition, resistance of the p-type cladding layer undesirably increases. These problems become more serious as the band gap of the active layer increases, namely, as the emission wavelength becomes shorter.
Furthermore, in conventional GaN semiconductor lasers, although the p-type Al0.2Ga0.8N cap layer 108 is interposed between the active layer 107 and the p-type GaN optical guide layer 109, the p-type Al0.2Ga0.8N cap layer 108 with a high Al composition is difficult to grow and decrease in resistance as mentioned above. In due course, there were also problems such as adverse influences to the laser property from an increase of electric resistance by the p-type Al0.2Ga0.8N cap layer 108.
Moreover, In conventional GaN semiconductor lasers, since there is a difference in lattice constant between the sapphire substrate 101 and the GaN semiconductor layers forming the laser structure, the GaN buffer layer 102 is grown on the sapphire substrate 101 at a low temperature, and it is crystallized when GaN semiconductor layers are grown thereon for the purpose of improving the quality of the GaN semiconductors grown on the GaN buffer layer 102. However, even when the GaN buffer layer 102 by low-temperature growth is used, there is a limit in density of defects which can be decreased.
It is therefore an object of the invention to provide a high-quality, high-performance semiconductor light emitting device using nitride III-V compound semiconductors, which can be reduced in threshold current density and operation voltage and can be shortened in emission wavelength down to the ultraviolet region.
Another object of the invention is to provide a semiconductor device using nitride III-V compound semiconductors with a high band gap, having excellent electric property and optical property.
To solve the problems contained in the conventional techniques, the Inventor made studies with every efforts. Its outline is explained below.
B-based semiconductors such as BN containing boron (B) as a group III element are stable in the sense of energy, strong against high energy light such as ultraviolet light; for example, and are hopeful for the future use. In particular, with nitride III-V compound semiconductors containing B, an increase in band gap by addition of B can be expected. Further, according to the knowledge of the Inventor, it has been confirmed that II-VI compound semiconductors and other III-V compound semiconductors are more easily p-typed as the covalent bond diameter of positive ions decreases (Hiroyuki Okuyama, Akira Ishibashi, Applied Physics 65,687(1996), herein after called Document 1). Therefore, analogically inferring from Document 1, it is considered that relatively high p-type carrier concentrations can be obtained with nitride III-V compound semiconductors containing B, in which the covalent radius of positive ion elements is reduced by addition of B.
Here is taken BpAlqGarInsN (0 less than pxe2x89xa61, 0xe2x89xa6q less than 1, 0 less than r less than 1, 0xe2x89xa6s less than 1, p+q+r+s=1) as a nitride III-V compound semiconductor containing B, and reviews are made on the use of this BpAlqGarInsN as a material of a semiconductor layer forming a light emitting structure.
In semiconductor light emitting devices, in general, it is considered desirable to make layers from the active layer to the cladding layer by using direct transition type semiconductors, taking influences of optical absorption into consideration. However, since BN is an indirect transition type semiconductor, for making mixed crystals with other direct transition type semiconductors, such as GaN and AlN, the range of composition of B must be fixed. FIG. 2 shows relations between lattice constants and energy gaps of typical GaN semiconductors. Shown in FIG. 2 are minimum energy gaps of r points and minimum energy gaps other than r points of GaN, AlN, InN and BN. If the Vegard law that a lattice constant linearly changes with composition ratio holds, the minimum of the wurtzite structure appears approximately at L point. Comparing them and calculating the intersection between direct transition and indirect transition from the interpolation, BAlN becomes an indirect transition type semiconductor with B composition not less than 10%, and BGaN becomes an indirect transition type semiconductor with B composition not less than 30%. In other words, in order to ensure that BpAlqGarInsN be of an indirect transition type, B composition not larger than 0.3, or more preferably not more than 0.1 will be acceptable.
Further, since BpAlqGarInsN enables an increase of the band gap by addition of B as explained above, it will be suitable as the material of the cladding layer which is desired to have a large band gap. Additionally, since BpAlqGarInsN can be more easily increased in p-type carrier concentration than AlGaN, it is advantageous for reducing the resistance of the p-type cladding layer as well. However, when growth of BpAlqGarInsN is taken into account, it is difficult to determine optimum growth conditions because BN and InN are extremely different in vapor pressure. Therefore, from the viewpoint of facilitating the growth, it would be desirable not to conduct addition of B and addition of In to a common layer simultaneously. Further, since quinary mixed crystals, in general, are difficult to control their growth, it is desirable to use quaternary or less mixed crystals for making the active layer that must be strictly controlled in composition. Especially, for selection of the material of the active layer, it should be taken into account that nitride III-V compound semiconductors containing B may deteriorate in optical property depending upon growth conditions, and nitride III-V compound semiconductors containing Al are liable to be oxidized and weak in optical damages. This is the case also for the optical guide layer. The same consideration is required also for the optical guide layer.
Furthermore, BpAlqGarInsN is considered hopeful as the material of buffer layers and cap layers of GaN semiconductor lasers. When BpAlqGarInsN is used as the buffer layer, the attachment coefficient of B is good, and the thermal stability is improved. Therefore, its growth temperature may be higher than that of a GaN buffer layer. Additionally, because of its small lattice constant, BN exhibits a firm coupling and allows less defects to enter. Therefore, when BpAlqGarInsN is used as the buffer layer, the density of defects of the buffer layer can be reduced, and also the density of defects of a semiconductor layer grown thereon can be reduced as well. Further, if the growth temperature is 1000xc2x0 C., equilibrium vapor pressure decreases from InN to GaN, AlN and BN in this order. Therefore, in order to prevent evaporation of In, the use of BN mixed crystal will be more effective.
As reviewed above, adequately using nitride III-V compound semiconductors containing B to make semiconductor layers forming a light emitting structure is considered effective to solve the problems involved in the conventional techniques.
In case of stacking a plurality of nitride III-V compound semiconductor layers containing B and those not containing B, differences in lattice constant among those layers must be taken into consideration. FIG. 3 shows relations between band gaps of representative GaN semiconductors and their lattice mismatches relative to GaN. Covalent coupling radius of B is 0.88 xc3x85, and smaller than covalent coupling radii of Ga and Al (1.26 xc3x85) and covalent coupling radius of In (1.44 xc3x85). Therefore, as shown in FIG. 3, a layer containing B decreases in lattice constant as the ratio of B increases, and when it is stacked on or under a layer not containing B, a tension force is applied thereto. To solve this problem, the Inventor made researches and invented the following techniques. The first technique is making a mask having an aperture on a substrate and selectively growing semiconductor layers on the substrate exposed in the aperture. The second technique is making a mesa portion on a substrate and selectively growing semiconductor layers on the mesa portion. In these first and second techniques, semiconductor layers grow in form of a trapezoid on the growth region of the substrate, and the semiconductor layers decreases in grown area as they become away from the substrate. Therefore, cracks can be prevented.
In addition to those researches, the Inventor made further researches about optimization of the laser structure and the growth condition to improve the property of GaN semiconductor lasers.
That is, the Inventor has known from experiments conducted heretofore that an excellent laser device capable of continuous oscillation can be obtained if the band gap difference between the cladding layer and the active layer is 500 meV at the least. The Inventor also reviewed about changes in threshold current density with changes in band gap difference between the cladding layer and the active layer. FIG. 4 shows a result of the measurement. In samples used in this experiment, the stripe size was 4 xcexcmxc3x971 mm, and the band gap difference between the cladding layer and the active layer was changed by changing the Al composition of the p-type AlGaN cladding layer. It is known from FIG. 4 that, when the band gap difference between the cladding layer and the active layer is equal to or larger than 500 meV, the threshold current density is reduced.
Further, using GaN semiconductor lasers as shown in FIG. 1, the Inventor reviewed changes in voltage with changes in Al composition of the cladding layer by supplying a current of 1 kA/cm2 in current density between electrodes. FIG. 5 shows a result of the measurement. It is known from FIG. 5 that an increase in voltage is observed when the Al composition of the cladding layer becomes 0.06 and higher, and the voltage greatly increases when the Al composition of the cladding layer becomes 0.1 and higher. Although this is the phenomenon about the Al composition of the cladding layer, the same can be said also about the B composition.
The result of the experiment shows that, from the standpoint of reducing the threshold current density of a GaN semiconductor laser, the band gap difference between the cladding layer and the active layer should be preferably 500 meV or more, and from the standpoint of reducing the operation voltage, the Al composition and the B composition of the cladding layer should be preferably 0.1 or less, and more preferably 0.06 of less.
Regarding growth conditions, in particular, the growth temperature, it is considered that nitride III-V compound semiconductors including In such as GaInN, in general, are grown at a temperature around 600xc2x0 C. through 800xc2x0 C., taking the problem of decomposition of InN into consideration, but nitride III-V compound semiconductors not including In, such as GaN and AlGaN are crystallographically better when grown at higher temperatures. This has been confirmed also through experiments conducted by the Inventor. FIG. 6 shows a result of measurement of the surface roughness of GaN films grown on substrates by MOCVD. The surface roughness was reviewed by investigating level differences in every 10 xcexcm squared areas of GaN films and standardized by putting minimum values as 1. It is known from FIG. 6 that GaN films grown at temperatures not lower than 1000xc2x0 C. are better in surface evenness than those grown at temperatures from 600 to 800xc2x0 C. Furthermore, for growth of nitride III-V compound semiconductors containing B, it is considered preferable to use a pressurized MOCVD apparatus as disclosed in Japanese Laid-Open Publication No. hei 9-168853 because BN simple substance tends to be produced under high pressures.
The present invention has been made through those researches by the Inventor.
According to the first aspect of the invention, there is provided a semiconductor device having a plurality of semiconductor layers made of nitride III-V compound semiconductors, comprising:
the plurality of semiconductor layers including at least one layer made of a nitride III-V compound semiconductor containing B within the range of B composition not higher than 0.3.
According to the second aspect of the invention, there is provided a semiconductor light emitting device having a light emitting structure composed of a plurality of semiconductor layers which are made of nitride III-V compound semiconductors, comprising:
the plurality of semiconductor layers, which constitutes the light emitting structure, including at least one layer made of a nitride III-V compound semiconductor containing B within the range of B composition not higher than 0.3.
In the present invention, each nitride III-V compound semiconductors are composed of at least one kind of group III element selected from the group consisting of Ga, Al, In, B and Tl, and one or more group V elements which include at least N and may additionally includes As or P. Si, for example, is used as the n-type impurity introduced into the nitride III-V compound semiconductors, and Mg, Zn or Be, for example, is used as the p-type impurity.
In the present invention, a pressurized metal organic chemical vapor deposition apparatus is preferably used for growing layers of nitride III-V compound semiconductors containing B.
In the present invention, B composition of layers made of III-V compound semiconductors containing B is preferably 0.1 or less.
In the present invention, layers of nitride III-V compound semiconductors containing B are preferably made of BpAlqGarInsN (where 0 less than pxe2x89xa60.3, 0xe2x89xa6q less than 1, 0 less than r less than 1, 0xe2x89xa6s less than 1, p+q+r+s=1).
In the first aspect of the invention, for the purpose of minimizing cracks and ensuring a good optical property, the plurality of semiconductor layers are preferably made by forming a mask having an aperture on a substrate and then selectively growing nitride III-V compound semiconductors on the substrate exposed in the aperture, or by forming a mesa portion on a major surface of a substrate and then selectively growing nitride III-V compound semiconductors on the mesa portion. Similarly, in the second aspect of the invention, for the purpose of minimizing cracks and ensuring a good optical property, the plurality of semiconductor layers forming the light emitting structure are preferably made by making a mask having an aperture on a substrate and then selectively growing nitride III-V compound semiconductors on the substrate exposed in the aperture, or by forming a mesa portion on a major surface of a substrate and then selectively growing nitride III-V compound semiconductors on the mesa portion.
In the second aspect of the invention, the light emitting structure preferably includes a structure including an active layer interposed between a first cladding layer and a second cladding layer, and more preferably, it additionally includes a first optical guide layer and a second optical guide layer interposed between the first cladding layer and the active layer and between the active layer and the second cladding layer, respectively.
In a first preferable combination of the material of the first and second cladding layers and the material of the active layer in the second aspect of the invention, the first and second cladding layer are made of BpAlqGarInsN (where 0 less than pxe2x89xa60.3, 0xe2x89xa6q less than 1, 0 less than r less than 1, 0xe2x89xa6s less than 1, p+q+r+s=1), and the active layer is made of AlxGayInzN (where 0xe2x89xa6x less than 1, 0xe2x89xa6y less than 1, 0 less than zxe2x89xa61, x+y+z=1). From the viewpoint of easier growth, the first and second cladding layers are more preferably made of BpAlqGarN (where 0 less than pxe2x89xa60.3, 0xe2x89xa6q less than 1, 0 less than r less than 1, p+q+r=1). The first and second optical guide layers are made of AlaGabIncN (where 0xe2x89xa6a less than 1, 0 less than bxe2x89xa61, 0xe2x89xa6c less than 1, a+b+c=1), and more preferably made of AlaGabN (where 0xe2x89xa6a less than 1, 0 less than bxe2x89xa61, a+b=1) In this first combination, since the first and second cladding layers contain B, their band gaps can be increased. Since the active layer and the optical guide layers are made of nitride III-V compound semiconductors which are quaternary or less mixed crystals not containing B, they exhibit a good growth controllability and a good optical property. When B is introduced into the active layer and the optical guide layers if so desired, their band gaps can be increased.
In a second preferable combination of the material of the first and second cladding layers and the material of the active layer in the second aspect of the invention, the first and second cladding layers are made of BpAlqGarInsN (where 0xe2x89xa6p less than 0.3, 0xe2x89xa6q less than 1, 0 less than r less than 1, 0xe2x89xa6s less than 1, p+q+r+s=1), and the active layer is made of BxGayInzN (where 0xe2x89xa6xxe2x89xa60.3, 0xe2x89xa6y1, 0 less than zxe2x89xa61, x+y+z=1). From the viewpoint of easier growth, the first and second cladding layers are more preferably made of BpAlqGarN (where 0 less than pxe2x89xa60.3, 0xe2x89xa6q less than 1, 0 less than r less than 1, p+q+r=1). The first and second optical guide layers are made of BaGabIncN (where 0xe2x89xa6axe2x89xa60.3, 0 less than bxe2x89xa61, 0xe2x89xa6c less than 1, a+b+c=1), and more preferably made of BaGabN (where 0xe2x89xa6a less than 1, 0 less than bxe2x89xa61, a+b=1). In this second combination, since the first and second cladding layers contain B, their band gaps can be increased. Since the active layer and the optical guide layers are made of nitride III-V compound semiconductors which are quaternary or less mixed crystals not containing Al, their optical damage levels can be increased. When B is introduced into the active layer and the optical guide layers if so desired, their band gaps can be increased.
In a third preferable combination of the material of the first and second cladding layers and the material of the active layer in the second aspect of the invention, the first and second cladding layer are made of BpAlqGarN (where 0 less than pxe2x89xa60.3, 0xe2x89xa6q less than 1, 0 less than r less than 1, p+q+r=1), and the active layer is made of BxAlyGazN (where 0xe2x89xa6xxe2x89xa60.3, 0xe2x89xa6y less than 1, 0 less than zxe2x89xa61, x+y+z=1). The active layer, first cladding layer and second cladding layer are grown at temperatures not lower than 1000xc2x0 C. The active layer is more preferably made of BxGazN (where 0xe2x89xa6xxe2x89xa60.3, 0.7xe2x89xa6zxe2x89xa61, x+z=1), AlyGazN (where 0xe2x89xa6y less than 1, 0 less than zxe2x89xa61, y+z=1), or GaN. The first and second optical guide layers are made of BaAlbGacN (where 0xe2x89xa6axe2x89xa60.3, 0xe2x89xa6b less than 1, 0 less than c less than 1). In this third combination, since the first and second cladding layers contain B, their band gaps can be increased. Additionally, when B and/or Al is introduced into the active layer and the optical guide layers if so desired, their band gaps can be increased. Further, since the active layer, first and second cladding layers, first and second optical guide layers are made of nitride III-V compound semiconductors not containing In, they can be grown at temperatures not lower than 1000xc2x0 C., and therefore improved in crystalline property.
In the second aspect of the invention, if any of the first to third combinations is used as the combination of the material of the first and second cladding layers and the material of the active layer, the band gap difference of the first and second cladding layers from the active layer is preferably 500 meV or more for the purpose of reducing the threshold current density. Especially in the third combination, difference between the B composition of the first and second cladding layers and the B composition of the active layer is within 5%. The difference of 5% between B compositions corresponds to the difference of 500 meV between band gaps.
In the second aspect of the invention, if any of the first to third combinations is used as the combination of the material of the first and second cladding layers and the material of the active layer, the Al composition or B composition of the first and second cladding layers is 0.1 or less, and more preferably 0.06 or less.
In the second aspect of the invention, if the plurality of semiconductor layers forming the light emitting structure are stacked on the substrate via a buffer layer, BpAlqGarInsN (where 0 less than pxe2x89xa61, 0xe2x89xa6q less than 1, 0xe2x89xa6r less than 1, 0xe2x89xa6s less than 1, p+q+r+s=1) may be used as the buffer layer. In this case, if the composition is chosen to ensure that the band gap of the buffer layer is larger than the band gap of the active layer, it is possible to take out the light from the active layer from the substrate side.
In the second aspect of the invention, in the case where a cap layer is provided near the active layer to prevent evaporation of In, BpAlqGarInsN (where 0 less than pxe2x89xa61, 0xe2x89xa6q less than 1, 0 less than r less than 1, 0xe2x89xa6s less than 1, p+q+r+s=1) may be used as the material of the cap layer.
According to the first and second aspects of the invention having the above-summarized configurations, by introducing B into desired one of a plurality of semiconductor layers within a range making a direct transition type, it is possible to increase its band gap, increase its p-type carrier concentration, and so on, while realizing a good optical property in the layer.
In particular, according to the second aspect of the invention, when B is introduced into the cladding layer among the plurality of semiconductor layers forming the light emitting structure, it is possible to increase the band gap of the cladding layer and thereby reduce the threshold current density, and it is advantageous also for realizing shorter emission wavelengths.
Moreover, when the buffer layer interposed between the substrate and semiconductor layers forming the light emitting structure contains B, it is possible to improve the thermal stability of the buffer layer and thereby reduce the defect density of the semiconductor layers formed thereon, and it is therefore possible to improve the optical property of the semiconductor layers.
Furthermore, when the cap layer formed near the active layer to prevent evaporation of In contains B, not only improved is the In evaporation preventing function, bu also lowered is the resistance of the cap layer. Therefore, the use of the cap layer contributes to preventing an increase of the operation voltage.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.